1. Field of the Invention
This invention relates to a display device. More particularly, it relates to a display device adapted to highly efficiently take out light emitted from electroluminescent (EL) elements that operate as so many display pixels to the outside.
2. Related Background Art
Display devices comprising a plurality of EL elements arranged two-dimensionally on a same substrate are known. However, in any known display devices, the ratio of the quantity of light that can be externally taken out to the total quantity of light emitted from each of the EL elements is not very large.
FIG. 1 of the accompanying drawings is a schematic cross sectional view of an EL element of a known display device, illustrating its basic structure. Referring to FIG. 1, the EL element 600 is formed by sequentially laying a transparent electrode 520, an electroluminescent (EL) layer 510 and a reflector electrode 500 on a transparent substrate 550 in the above mentioned order. Light emitted from the EL layer 510 is totally reflected at the interface B1 of the transparent substrate 550 and the transparent electrode 520 and the interface B2 of the transparent substrate 550 and ambient air. If the refractive index of the transparent electrode 520 is 1.8 and that of the transparent substrate 550 is 1.5, the quantity of light confined within the EL element 600 due to total reflection at the interface B1 is about 51% of the total quantity of light emitted from the EL element 600. On the other hand, the quantity of light confined within the EL element 600 due to total reflection at the interface B2 is about 32% of the total quantity of light emitted from the EL element 600. Therefore, the quantity of light that can be externally taken out from the transparent substrate 550 is only about 17% of the total quantity of emitted light.
Meanwhile, Optics Letters, Mar. 15 (1997) pp. 396 to 398, discloses an EL element realized by adding a transparent member having a trapezoidal cross section to the above described basic structure, from which light can be taken out at an improved efficiency. FIG. 2 of the accompanying drawings is a schematic cross sectional view of such an EL element. In FIG. 2, the components that are same as those of FIG. 1 are denoted respectively by the same reference symbols.
Referring to FIG. 2, the EL element 600 has a sandwich structure where an EL layer 510 is sandwiched between a reflector electrode 500 and a transparent electrode 520. The EL element 600 is laid on a transparent member 540 having a trapezoidal cross section and formed on a transparent substrate 550. When such EL elements are applied to a two-dimensional display device, the transparent member 540 of each EL element is typically realized in the form of a frustum of quadrangular pyramid. Then, a reflection film 530 is formed on the slopes of the transparent member 540.
With an EL element 600 having a configuration as shown in FIG. 2, no total reflection takes place at the interface 1 when the refractive index of the transparent member 540 is made greater than that of the transparent electrode. On the other hand, if total reflection occurs at the interface 2 of the transparent substrate 550 and ambient air, light I2 totally reflected by the interface 2 is reflected again by the reflection film 530 and taken out of the transparent substrate 550 into ambient air. Similarly, if total reflection occurs at the interface 3 of the transparent member 540 and the transparent substrate 550, light I3 totally reflected by the interface 3 is reflected again by the reflection film 530 and taken out of the transparent member 540. It may be needless to note that light emitted from the EL layer 510, transmitted through the transparent electrode 520 and directly reflected by the reflection film 530 goes out of the transparent substrate 550 into ambient air. Therefore, the above described arrangement allows light emitted from the EL layer 510 to be highly efficiently taken out to the outside.
However, if the reflection film 530 formed on the slopes of the transparent member 540 is made of metal in the above described EL element, it needs to be formed so as not to contact the transparent electrode 520 and the reflector electrode 500. It is not easy to form such a reflection film. Additionally, although not shown in FIG. 2, the sandwich structure sandwiching the EL layer 510 needs to be covered by a protection film in order to prolong the service life of the EL element 600. Then, such a protection film has to be prepared independently from the process of manufacturing the transparent member 540 to increase the number of total manufacturing steps and baffle effects for reducing the manufacturing cost.